Sen—Salty liquid water may exist close to the surface of Mars, according to a new study of data collected by NASA's Curiosity rover.

The rover has found salty perchlorate compounds in the Martian soil which, in the right conditions, can absorb water vapour from the atmosphere and lower the freezing point of water, meaning that liquid water in the form of brine (concentrated salt water) can form both at the surface and a few centimetres below it.

This process is only likely to occur when the air on Mars is at its coldest, at night or during cold winter mornings.

"Perchlorates are not only oxidants, but they also form highly hygroscopic (water-absorbing) salts and are strong freezing-point depressors when added to water," Professor Morten Bo Madsen, a member of Curiosity's science team from the Niels Bohr Institute in Copehagen, Denmark, told Sen.

After sunset, some water vapour condenses on the planet's surface as frost. The freezing point of the water is lowered when absorbed by perchlorates in the soil, such as calcium perchlorate, thus turning the icy frost into a liquid. As the soil is porous, the now-liquid water can seep down and precipitate elsewhere below the surface.

"This can explain mobility of salts just below the surface, and with this study we now know that this can occur presently and hence is not just a relic from the past," said Morten Bo Madsen. However, this liquid water is transient, forming within the uppermost 5cm (2in) of the surface overnight, and drying out again at sunrise.

The study is based on data collected by Curiosity's Rover Environmental Monitoring Station (REMS), Sample Analysis at Mars (SAM) and Dynamic Albedo of Neutrons (DAN) instruments over the course of a full Martian year (1.88 Earth years). This has given scientists the largest environmental data set ever recorded in-situ on the Red Planet.

"This is not about the detection of salts, but about the impact these salts have on the possibility to have liquid water on Mars," Dr Javier Martin-Torres, senior research scientist at the Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR) in Granada, Spain, and lead author of the paper, told Sen. "If you look at the phase diagram of pure water, you will find at the pressure and temperatures of Mars it is impossible to have liquid water on the surface. This has been well known for a long time."

Perchlorates were first discovered by NASA's Phoenix spacecraft back in 2008, although their presence has been speculated since experiments on board NASA's Viking crafts first detected an oxidising compound in the Martian soil in 1977. The perchlorate ion, made of one chlorine atom and four oxygen atoms, typically combines with elements such as magnesium, calcium or iron to form a salt.

"The perchlorate is only present in very small amounts (between 0.5 and 1 per cent by weight), as is the case on Earth," said Professor Morten Bo Madsen. "Perchlorates are rare in nature, but do occur at very low concentrations in very arid areas such as the Atacama desert."

Dr Martin-Torres said: "This is the first time that we find conditions for the formation of brines on Mars. There have been hypotheses and laboratory studies supporting this possibility, but even these works were pointing to this only occurring at high altitudes.

"The temperatures for formation are very low, and only reached (at least in the Equator, where Curiosity is) at night time, and especially in winter-time. It is really a surprise that we find brine conditions at the Equator, which is the driest and hottest region of the planet, but the fact that we find them there tells us that brines must be everywhere in the planet where there are perchlorates."

The combination of instruments onboard Curiosity (more formally known as the Mars Science Laboratory, or MSL) are not designed to directly detect liquid water, or indeed life, but rather their purpose is to evaluate the conditions that lead to habitability.

"The transition of the existing salts within the soil, from hydrated or frozen states, into small liquid droplets could only be directly detected either by measuring electrical conductivity changes or calorimetric changes of that droplet or tiny salt patch within the soil," says Martin-Torres. "This kind of contact science requires a sub-surface probe and instruments that currently do not exist within the MSL platform."

He concludes: "In general, the findings of this study have broad implications for studies on the availability and the history of water on Mars, for preserving plausible organic products, for studying the corrosive interaction of these brines with spacecraft materials, and, of course, for other geological processes related to water and climate on Mars today."

The results of the new study have been published in the Journal Nature Geoscience.